<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        // 字符串  数字 不尔   
        //  undefined  null
        // undefined
        var str = "abcdefg";
        console.log(str);
        function fn(str){
            var a = new String(str);
            console.log(a);
            var arr = a.split("");
            return arr;
        }
       var arr = fn(str);
       console.log(arr);
    </script>
</body>
</html>